(1) Field of the Invention
The present invention relates to a transmission particularly for an agricultural tractor, which transmission includes change speed mechanisms for transmitting engine output in various speeds to ground-engaging wheels and a working implement.
(2) Description of the Prior Art
Known tractor transmissions in most cases include a main change speed mechanism, an auxiliary change speed mechanism and a rear differential mechanism housed in a single transmission case. Therefore, when modifying the specifications of the change speed mechanisms, for example increasing the number of speeds provided thereby, it is necessary to change the design of the transmission case as well as the construction of the change speed mechanisms. This means that the specifications of the change speed mechanisms cannot be changed by employing a simple additional unit for models already manufactured and sold. One prior art example is known from U.S. Pat. No. 4,273,007 in which the transmission case comprises a front case housing a main change speed mechanism and a rear case housing a rear differential mechanism, and an auxiliary case housing an auxiliary change speed mechanism is attached to an opening defined on a top wall of the front case. U.S. Pat. No. 4,326,597 discloses another prior art example in which an auxiliary case housing an auxiliary change speed mechanism is interposed between a front case and a rear differential mechanism. These transmissions meet the requirement for modification to the specifications in that the entire auxiliary change speed mechanism may be changed as necessary. They may be of advantage to the maker for the purpose of modifying models in the manufacturing process, but the changing operation is too troublesome for the user to carry out and is hardly impracticable for him. It is an excessive burden on the maker to provide the user with various types of auxiliary change speed mechanism as options in order to meet the diverse needs of the user since it would oblige the maker to manufacture various types of large parts in small quantities.